1. Field of the Invention
Aspects of the present invention relate to a method and apparatus to control a position in which data is to be recorded, and more particularly, a method and apparatus to control a position in which data to be recorded on a holographic data recording medium.
2. Description of the Related Art
Technology for storing information by using holograms has become widely used. In such technology, information is stored in an optical interference pattern on an inorganic crystal that is photosensitive or on a photosensitive material such as a photo polymer. Optical interference patterns are formed by using two coherent laser beams. In other words, interference patterns are formed when a reference light and a signal light having different paths interfere with each other, cause chemical or physical changes in a photosensitive information storage medium, and are recorded thereon. During reproduction, a reference light that is similar to the reference light used to record information is irradiated on the interference patterns recorded on the information storage medium so as to reproduce information from the recorded interference patterns. This causes diffraction due to the interference patterns and, as such, the signal light is restored and information is reproduced.
Examples of technology to store information by using holograms include a volume holography method by which information is recorded and/or reproduced in one page units by using volume holography, and a micro-holography method by which information is recorded and/or reproduced in single bit units by using micro-holography. In the volume holography method, a large amount of information can be processed simultaneously. However, since an optical system should be very precisely adjusted, it is not easy to adopt the method in an apparatus to store information for general consumers.
In the micro-holography method, two condensed light beams interfere with each other at a particular point of focus and thus form fine interference patterns (micro-holograms) on a plane of an information storage medium. A plurality of such interference patterns are recorded on the information storage medium to form a plurality of recording layers. The recording layers are superimposed in a depth direction of the information storage medium to form a multi-layer structure so that information can be recorded three-dimensionally on the information storage medium.
Basically, in the micro-holography method, information is recorded in a plurality of recording layers in the depth direction of the information storage medium, thereby increasing a recording capacity of the information storage medium. In a multi-layer optical disk, such as a Blu-ray disk (BD), a reflective layer exists in each of the recording layers. The plurality of recording layers are distinguished according to the intensity of a signal of reflected light and the polarity of the signal, and an optical focus is formed in a desired recording layer.
However, unlike in a conventional optical disk, in the information storage medium used in the micro-holography method, a reflective layer does not exist in each of the recording layers. Specifically, when a reflective layer exists in each of the recording layers of the information storage medium, efficiency of recorded light is degraded due to the reflective layer. Thus, information cannot be recorded in a plurality of layers in the depth direction of the information storage medium, and a recording capacity may not be increased.
Thus, in the information storage medium used in the micro-holography method, a reflective layer does not exist in each of the recording layers. As such, it is not easy to form an optical focus in a desired recording layer of the information storage medium. When an optical focus is not formed on the desired recording layer, a distance between the recording layers may not be uniform. As a result, there may be a change in crosstalk between adjacent layers, and the performance of a reproduction signal in each of the recording layers may be different. Furthermore, the optical focus may be formed in a portion that is far away from the recording layers, and recording may not be appropriately performed even when recording starts.